284 MORPHOLOGY AND EVOLUTION
wards and forwards on the dorsal aspect of the insula; its dorsal lip may be operculated, thereby depressing the insula. The other sulcus, namely, the fronto-orbital (F.O.S.), which may be very deep, bounds the insula below and anteriorly. These arrangements round the anterior part of the insula distinguish the Gibbons from the Monkeys, and ally them to the Orang, the Chimpanzee, and the Gorilla.
On the excavated inferior surface of the frontal lobe lies the linear orbital sulcus (O.S.), or the triradiate system of orbital sulci. And the infero-lateral parts of the occipital lobe exhibit the inferior occipital sulcus, which may be connected to the posterior extremity of the middle temporal sulcus; it may also be united to the occipito-temporal sulcus.
The latter, which is formed to relieve the tension of the expanding cerebral cortex, may be large, its increase in size taking place at the expense of the collateral sulcus.
The mesial aspect of the hemisphere exhibits in all specimens a very well marked caUoso-marginal or intercalary sulcus whose upturned posterior extremity appears on the outer surface of the brain. It is a deep sulcus. The collateral sulcus is frequently seen to consist of two sulci.
In many brains the sulci are deep and the gyri are so
pronounced that they are accommodated in deep depressions on the inner aspect of the skull.
In the hind brain the condition of the floccular parts of the cerebellum is of interest, for it is intermediate between that present in the brain of a Monkey and that in the higher Simiidae.